fix(server): use of inconsistent methods for determining origin

[james-d-elliott]

This unifies the methods to obtain the X-Forwarded-* header values and provides logical fallbacks. In addition, so we can ensure this functionality extends to the templated files we've converted the ServeTemplatedFile method into a function that operates as a middlewares.RequestHandler.

[authelia]

Artifacts

These changes are published for testing on Buildkite, DockerHub and GitHub Container Registry.

Docker Container

• docker pull authelia/authelia:fix-origin-processing
• docker pull ghcr.io/authelia/authelia:fix-origin-processing

[codecov]

Codecov Report

Merging #2848 (648a7c9) into master (7775d2a) will decrease coverage by 0.12%.
The diff coverage is 80.20%.

@@            Coverage Diff             @@
##           master    #2848      +/-   ##
==========================================
- Coverage   63.89%   63.77%   -0.13%     
==========================================
  Files         217      217              
  Lines       10645    10660      +15     
  Branches      270      270              
==========================================
- Hits         6802     6798       -4     
- Misses       3349     3363      +14     
- Partials      494      499       +5     

Flag	Coverage Δ
backend	67.48% <80.20%> (-0.16%)	⬇️
frontend	41.73% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/server/template.go	47.56% <37.50%> (ø)
internal/middlewares/authelia_context.go	69.66% <83.13%> (-0.74%)	⬇️
internal/middlewares/strip_path.go	100.00% <100.00%> (ø)
internal/server/server.go	82.09% <100.00%> (ø)
internal/handlers/handler_register_u2f_step1.go	15.62% <0.00%> (-12.50%)	⬇️
internal/ntp/ntp.go	51.06% <0.00%> (-10.64%)	⬇️
internal/handlers/handler_sign_u2f_step1.go	7.24% <0.00%> (-5.80%)	⬇️

[nightah]

Small issue, otherwise LGTM.